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R. D. M coY 2,795,653

VACUUM TUBE VOLTMEITER AMPLIFIER CIRCUIT 2 Sheets-Sheet 1 June 11, 1957Filed NOV. 12, 195} INVENTOR. EAWLEY D McCoY HTTO ENEYS June 11, 1957 p.c oy 2,795,653

VACUUM TUBE VOLTMETER AMPLIFIERCIRCUIT Filed NOV. 12. 1953 2Sheets-Sheet 2 INVENTOR. EAWLEY D Mc Gov INPUT HTTOENEYSS United StatesPatent VACUUM TUBE VOLTll/[ETER AMPLIFIER CIRCUIT Rawley D. McCoy,Bronxville, N. Y., assignor to Reeves Instrument Corporation, New York,N. Y., a corporation of New York Application November 12, 1953, SerialNo. 391,450

4 Claims. (Cl. 179-171) This invention relates to electronic amplifyingsystems and more particularly to a stable high gain direct currentamplifier ideally suited for applications requiring a high inputimpedance such as vacuum tube voltmeters and other control and measuringequipment.

While the general usefulness of this invention will become apparent asthe description proceeds, for simplicity, the invention will bediscussed in connection with its use as a vacuum tube voltmeter becauseof the high standards that must be met by electronic equipment in suchapplications. Vacuum tube voltmeters are used to measure both D. C. andA. C. potentials and are widely employed in the electronic field formeasurement of potentials in high impedance circuits. In order tomeasure the potential, for instance, in a high impedance circuit it isnecessary that the impedance of the measuring instrument be many timesthe impedance of the circuit in order to prevent any alteration of theoperating characteristics of the circuit tested which would produceerroneous readings. Furthermore, since test equipment must be calibratedagainst standards in order to attain high orders of accuracy, it isdesirable to provide a measuring instrument having a high degree ofstability or substantially zero drift over long periods of time. Thesefactors have presented a serious problem because the high impedanceinput requirements for a voltmeter which is usually of the order of ormore megohms tends to produce considerable instability. In addition, thedifficulty of attaining high impedance and stability is furthercomplicated by the need for high gain or amplification to facilitate themeasurement of minute potentials.

This invention provides a circuit and method of operation that not onlyoffers a solution of the foregoing problems but in addition provides arelatively simple system particularly suited for direct current vacuumtube voltmeter devices and other direct current amplifier applications.These ends are attained by means of a novel and improved circuit andarrangement of components which enables the utilization of inexpensivecomponents to produce stable high gain circuits and at the same timeenables the attainment of a very high input impedance and a low outputimpedance for operating a meter or other low impedance load.

Another important factor to be considered particularly in connectionwith measuring equipment is means to prevent possible overloading andconsequent injury of sensitive meters or other measuring instrumentsforming the indicator. Since it is desirable for vacuum tube voltmetersto measure voltages of the order of .1 volt to 1000 volts or more, awrong setting can produce considerable damage. Accordingly, it isanother object of the invention to provide a vacuum voltmeter that isarranged to provide very high sensitivities and at the same time protectthe indicating instrument against accidental damage.

Still another object of the invention is an electronic amplifier of highgain and stability wherein a portion of the output voltage isperiodically fed to the input to pro- 2,795,653 Patented June 11, 1957vide high input impedance and at the same time modulate the D. C.potential applied to the input.

A further object of the invention is an improved voltage amplifiercircuit employing a pentode vacuum tube wherein a gain substantiallyequal to the amplification factor is secured and linear amplifyingcharacteristics are obtained notwithstanding differences in thetransconductance of different tubes of the type for which the circuit isdesigned.

The above and other objects and advantages of this invention will becomemore apparent from the following description and accompanying drawingsillustrating one embodiment thereof.

In the drawings:

Fig. 1 is a simplified block diagram of the invention illustratingcertain aspects thereof; and

Fig. 2 is a circuit diagram of a vacuum tube voltmeter embodying theinvention.

Briefly, the invention utilizes an A. C. amplifier and a D. C. amplifierconnected in cascade with means for modulating the signal applied to theA. C. amplifier and demodulating its output so that the device willaccurately measure D. C. potentials. The modulating means in addi tionto modulating the input D. C. potential also provides feedback betweenthe output and input circuits in order to attain the necessary highinput resistance. These elements are illustrated in the block diagramFig. 1 which serves to explain certain of the principles of theinvention.

In this figure the A. C. amplifier is denoted by the numeral 10 whilethe D. C. amplifier is denoted by the numeral 12. These amplifiers arecoupled by a low-pass filter comprising a resistor 14 and a condenser 16connected between the output side of resistor 14 and ground. The inputcircuit of the A. C. amplifier 10 includes a series resistor R1 while ameter 18 is connected between the output of the D. C. amplifier andground. Since the basic input signal e is direct current it must bemodulated in some manner in order to enable its amplification by the A.C. amplifier 10. The amplified signal must then be demodulated in orderto register on the D. C. indicating instrument 18 connected in theoutput of the D. C. amplifier 12. These ends are attained through a newand improved circuit arrangement employing a singlepole double-throwvibrator interconnected with the A. C. amplifier 10 and the output ofthe D. C. amplifier 12 so that in addition to the modulation anddemodulation, the vibrator functions to attain a high input impedancefor the A. C. amplifier which is an essential requirement for vacuumtube voltmeters.

More particularly, the vibrator 20 includes a pair of fixed contacts 22and 23 with the contact 22 connected to the input of the A. C. amplifier10 and the contact 23 to the output side of that amplifier. The vibratorarmature 21 is connected to the junction of two series connectedresistors R2 and R which forms a voltage divider between the output ofthe D. C. amplifier 12 and ground. In actual practice the resistor R3 isvery small as compared with resistor R2 in order to get the desiredfeedback voltage to the input of the A. C. amplifier and provideeffective demodulation of the amplified A. C. signal. The vibrator isoperated by an electromagnetic coil 24 con nected to a suitable sourceof alternating current such as 60 cycle current that is commonlyavailable.

The input impedance may be computed as follows:

When the vibrator arm 21 is closed on contact 22 the voltage at theinput of the A. C. amplifier is v -LR! since the input and outputvoltages are in phase. When it is open the input voltage is e Thus asquare wave signal of an amplitude i R1 is fed to the A. C. amplifier.Therefore =i R1K (where K is the overall amplifier gain) Also R31 hen?)The factor 2 in the last equation above arises because the current ifiows only half the time. Moreover, practical considerations such as thefact that R3 must be low in comparison to R2 and that R2 must be muchsmaller than R1 (for the case of a current amplifier) permitssimplification of the impedance equation with relatively little errorto:

Actual input impedance computations based on an amplifier with anoverall gain of about 500, and resistor values of R1=500 kilohms, R2=l0kilohms and Rs=250 ohms produces an input impedance of about 16 megohms.It is quite apparent that this is not a limited value but that muchhigher impedances may be easily obtained by use of other values.Furthermore, the particular circuit and vibrator combination, in actualtests, provides substantially drift free operation over an eight hourperiod with .1 volt input providing substantially full scale deflectionon the meter 18.

It will be observed in Fig. 2 that the A. C. amplifier may comprise apentode amplifier tube 26 and a triode tube 27 connected as a cathodefollower while the D. C. amplifier 12 may be a single triode 28connected as a cathode follower with the meter 18 in the cathodecircuit.

Considering now the details of the circuits employed in the illustratedembodiment of Fig. 2, the input signal is applied to resistor R1 througha low pass input filter consisting of resistor 29 and condenser 30. Thisfilter reduces the effect of hum on the output D. C. current and alsoreduces feedback of noise from the amplifier to the input circuit.Following this filter and resistor R1 is a condenser 31 for coupling thesignal to the grid 32 of tube 26. The return for grid 32 is resistor 33connected from that grid to ground. The cathode 34 and suppressor 35 areinterconnected one with the other and to ground through the parallelarrangement of resistor 36 and bypass condenser 37 to provide thedesired bias for the tube. The screen grid 38 is bypassed to ground bycondenser 48 and is connected to the cathode 43 of tube 27 through aresistor 4811.

By connecting the screen 38 of the tube 26 to the cathode of tube 27, asmall amount of degenerative feedback is obtained and the operation oftube 26 is controlled so that tubes of different transconductances maybe used in the circuit without affecting the linearity and stability ofthe device. The plate circuit for tube 26 includes the plate 39 andseries connected resistors 40 and 41 to a source of potential such as+300 volts D. C.

The tube 26 is coupled to the cathode follower 27 by an improvedbootstrap circuit in order to realize the maximum gain of tube 26, andattain linear and highly stable operation. This is attained by directconnection of the plate 39 of tube 26 to the grid 42 of tube 27 andconnection of the cathode 43 through the coupling con- 4 denser 44 tothe junction of resistors 40 and 41. The plate 45 of tube 27 isconnected directly to +300 volts D. C. while the cathode 43 is returnedto ground through the cathode load resistor 46 and supplies the screenvoltage for tube 26. The components are adjusted so that both ends ofresistor 40 (which may be of the order of 500,000 ohms) are maintainedat the same A. C. potential in which case the gain of tube 27 will beclose to unity and constant current will tlow through resistor 40. Underthese conditions no signal voltage drop will appear across the internalplate resistance of tube 26 and the gain will closely approach the mu oramplification factor of the tube.

For example e muR garn= in 711+ R Where v is the plate resistance and Ris the load resistance. Tllfll] if 'y u rnuR The amplified A. C. signal.now appearing at the cathode 43 of tube 27 is fed through the seriesconnected condenser 47 and resistor 14 to the grid 49 of tube 28 whichis bypassed to ground through condenser 16. The plate 50 is connected tovolts D. C. through a resistor 51 and the cathode 53 is connected to 100v. D. C. through a potentiometer 54 and a resistor 55. The outputcircuit of tube 28 includes the meter 18 in series with a fixed resistor56 and an adjustable calibrating resistor 57 connected between themovable tap 54' on resistor 54 and ground. The vibrator 20 and resistorsR2 and R3 are connected as described in connection with Fig. l, the highside of resistor R2 being connected to the tap 54 on resistor 54.

As was discussed in connection with Fig. 1, the vibrator armature 21 inperiodically contacting the fixed contact 22 produces a square waveinput signal at the input to condenser 31. This signal uponamplification by tubes 26 and 27 appears at the cathode 43 of tube 27and is fed through condenser 47 and resistor 14 to tube 28. Howeversince resistor R3 is of the order of several hundred ohms and resistor14 about /2 megohm, when the armature 21 contacts the contact 23,substantially complete interruption of the signal occurs. Thisinterruption being in properly phased relationship with the generationof the square wave input signal results in rectification or demodulationthereof. In addition, the resistor 14 and condenser 16 act as a low-passfilter so that a filtered D. C. signal appears at the grid 49 of tube28. The output signal for actuation of the meter 18 then appears acrossthe cathode and plate load resistors 54, 55, 51 and the internalresistance of tube 28. The cathode follower tube 28 thus not onlyprovides a high impedance circuit for effective demodulation but also alow impedance output circuit for the meter and for attaining a lowimpedance circuit (R2 and Ra) for the feedback signal so that effectivemodulation also can be secured.

In the illustrated embodiment of the invention the meter 18 has its zeroposition at mid-scale to facilitate reading positive and negativevoltages without the need for an auxiliary switch. The meter istherefore connected between the tap 54' of resistor 54 which iscoordinated with resistors 51 and 55 so that a zero reading will besecured when no signal input is applied. The adjustable resistor 57 andfixed resistor 56 are selected so that the reading on the meter can beaccurately calibrated against a known input signal. The foregoing D. C.amplifier output and meter circuit has in addition to the advantagesenumerated, the further advantage of protecting the meter 18 againstdamage due to the accidental application of excesive signals to theinput of tube gain 26. By proper adjustment of resistor 51 incoordination with the dynamic plate resistance of tube 28, the positivecurrent flowing through the meter can be limited to a predeterminedmaximum value and similarly the negative current may be limited byproper choice of resistors 54 and 55.

By operating the tube 26 within the linear portions of its operatingcurve, the output voltage will be directly proportional to the inputsignal and the calibration of the device at one voltage willautomatically calibrate the device for all voltages. Multiple voltageranges may, of course, be provided by inserting an appropriate voltagedividing system either at the input or by changing the feedback byaltering the values of the resistors R2 and Rs.

As to the stability of the system above described, only one timeconstant outside of the A. C. amplifier is involved namely that ofresistor 14 and condenser 16 and actual experiment has indicated thatthis time constant does not vary sufliciently to affect the stability.It is desirable however that resistors R1, R2, R3, 55, 56 and 57 havelow temperature coefficients and it has been found that with goodcommercial components drifts of less than 2 millivolts will be obtainedfor periods of eight hours or more.

Although this invention has been described as a vacuum tube voltmeter,it is quite apparent that it may be used for other measuring and controlapplications requiring at least certain of the advantages offered bythis circuit. Moreover, many changes, modifications and alternations maybe made without departing from the true scope and spirit of theinvention.

I claim:

1. A feedback amplifier circuit comprising in combination, first andsecond electron tubes, said first electron tube having at least acathode, control electrode, screen electrode, and plate; said secondelectron tube having at least a cathode, control electrode, and plate;an input circuit coupled to said first electron tube for receiving anapplied input voltage, means coupling the plate of said first tube tothe control electrode of said second tube, first and second plate loadimpedance means coupled in series between the plate of said first tubeand the positive terminal of a source of potential, means coupling theplate of said second tube to the positive terminal of said source ofpotential, means coupling the cathode of said first tube to the negativeterminal of said source of potential, load impedance means coupling thecathode of said second tube to the negative terminal of said source ofpotential, a first feedback coupling means between the cathode of saidsecond tube and the junction of said first and second plate loadimpedance means, and second feedback coupling means between the cathodeof said second tube and the screen electrode of said first tube, saidsecond feedback coupling means providing a negative feedback path fromthe plate of said first tube through said second tube to said screenelectrode for stabilizing the gain of said first electron tube.

2. The feedback amplifier circuit as defined in claim 1 wherein saidinput circuit is adapted for coupling said applied input voltage betweenthe control electrode of said first tube and the negative terminal ofsaid source, and means adapted for coupling a utilization circuitbetween the cathode of said second tube and the negative terminal ofsaid source.

3. A stabilized amplifier system comprising in combination, a pentodeamplifier stage having an input circuit for receiving an applied voltageand having first and second series-coupled plate load impedances, acathode follower stage having an input and output circuit, meanscoupling the output voltage across said first and second seriescoupledanode load impedances to the input circuit of said cathode followerstage, means coupling the output voltage from the output circuit of saidcathode follower stage to the junction of said first and secondseriescoupled load impedances, and means directly coupling the outputvoltage from the output circuit of said cathode follower stage to thescreen grid tof said pentode amplifier for forming a negative feedbackloop between the output of said pentode amplifier stage through saidcathode follower stage to the screen grid of said pentode amplifierstage for stabilizing the gain of said pentode amplifier.

4. The stabilized amplifier system as defined in claim 3 wherein saidmeans directly coupling the output circuit of said cathode followerstage to the screen grid of said pentode amplifier includes a low-passfilter.

References Cited in the tile of this patent UNITED STATES PATENTS2,435,331 Street Feb. 3, 1948 2,459,730 Williams Jan. 18, 1949 2,517,863Froman Aug. 8, 1950 2,538,488 Volkers Jan. 16, 1951 2,615,064 StantonOct. 21, 1952 2,619,552 Kerns Nov. 25, 1952 2,685,000 Vance July 27,1954 FOREIGN PATENTS 630,780 Germany June 17, 1930 620,140 Great BritainMar. 21, 1949 646,581 Great Britain Nov. 22, 1950

